The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. About half of all dinoflagellates are photosynthetic, and these make up the largest group of algae aside from the diatoms. Some species, called zooxanthellae, are endosymbionts of marine animals and protozoa, and play an important part in the biology of coral reefs. Other dinoflagellates are colorless predators on other protozoa, and a few forms are parasitic.
Most dinoflagellates are unicellular forms with two dissimilar flagella. One of these extends towards the posterior, called the longitudinal flagellum, while the other forms a lateral circle, called the transverse flagellum. In many forms these are set into grooves, called the sulcus and cingulum. The transverse flagellum provides most of the force propelling the cell, and often imparts to it a distinctive whirling motion, which is what gives the name dinoflagellate refers to (Greek dinos, whirling).
Dinoflagellates have a complex cell covering called an amphiesma, composed of flattened vesicles, called alveoli. In some forms, these support overlapping cellulose plates that make up a sort of armor called the theca. These come in various shapes and arrangements, depending on the species and sometimes stage of the dinoflagellate. Fibrous extrusomes are also found in many forms. Together with various other structural and genetic details, this organization indicates a close relationship between the dinoflagellates, Apicomplexa, and ciliates, collectively referred to as the alveolates.
The chloroplasts in most photosynthetic dinoflagellates are bound by three membranes, suggesting they were probably derived from some ingested alga, and contain chlorophylls a and c and fucoxanthin, as well as various other accessory pigments. However, a few have chloroplasts with different pigmentation and structure, some of which retain a nucleus. This suggests that chloroplasts were incorporated by several endosymbiotic events involving already colored or secondarily colorless forms. The discovery of plastids in Apicomplexa have led some to suggest they were inherited from an ancestor common to the two groups, but none of the more basal lines have them.
Dinoflagellates have a peculiar form of nucleus, called a dinokaryon, in which the chromosomes are attached to the nuclear membrane. These lack histones and remained condensed throughout interphase rather than just during mitosis, which is closed and involves a unique external spindle. This sort of nucleus was once considered to be an intermediate between the nucleoid region of prokaryotes and the true nuclei of eukaryotes, and so were termed mesokaryotic, but now are considered advanced rather than primitive traits. In most dinoflagellates, the nucleus is dinokaryotic throughout the entire life cycle.
Most dinoflagellates are haploid, and reproduce primarily through fission, but sexual reproduction also occurs. This takes place by fusion of two individuals to form a zygote, which may remain mobile in typical dinoflagellate fashion or may form a resting cyst, which later undergoes meiosis to produce new haploid cells. Dinoflagellate cysts are found as common microfossils from the Triassic period onwards. A few are known from as far back as the Silurian, and it is likely that some of the older acritarchs also represent dinoflagellates.
Dinoflagellates sometimes bloom in concentrations of more than a million cells per mililitre. Some species produce neurotoxins, which in such quantities kill fish and accumulate in filter feeders such as shellfish, which in turn may pass them on to people who eat them. This is called a red tide, from the color the bloom imparts to the water. It should be noted that not all dinoflagellate blooms are dangerous. Bluish flickers visible in ocean water at night often come from blooms of bioluminescent dinoflagellates, which emit short flashes of light when disturbed.
The dinoflagellates were first defined by Otto BŁtschli in 1885 as the flagellate order Dinoflagellida. Botanists treated them as a division of algae, named Pyrrhophyta after the bioluminscent forms (Greek pyrrhos, fire). They have also been called the Dinophyta or Dinoflagellata. At various times the cryptomonads, ebriids, and ellobiopsids have been included here, but do not appear closely related.